Big jump in attenuation

My downstream attenuation on adsl2+ has always been around 34dB and with a 6dB noise margin I could sync at around 12Mbs. My true distance from the exchange is about 2.2 km and all the figures stack up. Earlier this year I migrated to Andrews and Arnold and my line stats were much the same and as you would expect even being moved to LLU with the talkTalk backhaul.

One day, the downstream attenuation jumped to about 50dB, with no change in the upstream attenuation but I continue to get much the same sync rate. AA support couldn't explain it but I would love to know what has caused it and why I continue to get the same sort of performance.

I am using a Billion 7800DXL router, but I get the same attenuation figures with other routers so it doesn't seem to be a function of how the attenuation is measured.